<!DOCTYPE html>
<style>
thead { break-inside: avoid; }
td { border: 1px solid black; padding: 5px; }
table { border-spacing: 0; }
</style>
<div style="columns: 2">
  <table>
    <thead>
      <tr><td>
        <div style="columns: 2">
          <table>
            <thead><tr><td>Inner</td></tr></thead>
            <tr><td style="height: 100px"></td></tr>
            <tr><td style="height: 100px"></td></tr>
          </table>
        </div>
      </td></tr>
    </thead>
    <tr><td style="height: 400px"></td></tr>
    <tr><td style="height: 400px"></td></tr>
  </table>
</div>
